Owing to the increasing share of renewable energies in the power generation infrastructure, there are fluctuations in the amount of current in the network. Said fluctuations must be compensated during power generation in order to ensure system stability in the network.
The fluctuations in the current in the network cannot be planned and are very marked in some cases. Consequently, there is a need for energy stores which can remove excess current from the network, or feed it into the network, given a shortage of current.
Secondary cells, in particular rechargeable batteries or accumulators, constitute one known option for storing and supplying current. They offer the option of storing electrical energy as chemical energy and supplying it again when required. Secondary cells further offer the advantage that the capacitance can be adapted by the use of so-called packages as required. Here, a package constitutes a combination of at least two secondary cells.
Secondary cells are, however, subject to aging processes which lead to a loss of capacitance. Said aging processes concern the charging and discharging of the secondary cell. The loss in capacitance is dependent, in particular, on temperature, time, current load, discharge depth and state of charge used when a secondary cell is operated or stored. The determination of the loss of capacitance is performed either directly by measurement—which is mostly impossible during operation—or by simulation with the aid of mathematical models. A simple relationship between charge throughput and loss of capacitance is made in said models. Said relationship can be described using formula 1.
                              Q          less                =                  B          ·                      exp            ⁡                          (                                                -                                      E                    a                                                  RT                            )                                ·                                    (                              I                *                t                            )                        z                                              formula        ⁢                                  ⁢        1            
In formula 1, R describes the universal gas constant, T temperature, I current and t time. Said parameters are fixed. B represents a prefactor, Ea the activation energy and z an exponent. Said parameters are adjustable. They must be adjusted to boundary conditions of each specific secondary cell by determining functional relationships between, for example, temperature, time, current load, discharge depth or current state of charge.
If the boundary conditions change during operation of the secondary cell, partial losses of capacitance are determined. The partial losses of capacitance are then added in order to determine the overall loss of capacitance. The relationship between the partial losses of capacitance is made in such a way that the charge throughput at the end of a partial loss of capacitance corresponds to the charge throughput at the beginning of the next partial loss of capacitance.
Said process is illustrated in FIG. 1. There, the loss of capacitance is plotted against charge throughput. Two functions which describe two different boundary conditions A and B are, furthermore, to be seen there. The loss of capacitance rises with increasing throughput for both boundary conditions. If the boundary conditions change, the changeover is made from the first function to the second function in the following way.
In a first step, the secondary cell is damaged from a first charge throughput a up to a second charge throughput b under boundary condition A. The boundary conditions change from A to B at a first final value 5. In this case, for further operation under boundary condition B the point of intersection is determined for the same charge throughput with the aid of the function for boundary condition B. The latter now serves as second initial value 6 for operating the secondary cell under boundary condition B.
The boundary conditions change from B to A after an operation from the second charge throughput b up to the third charge throughput c as far as the second final value 7. The determination of the third initial value 8 is performed in the same way for a constant charge throughput. A first partial loss of capacitance 9 and a second partial loss of capacitance 10 for the charge throughput ranges of a to b and b to c are read off at the y-axis and added.
It is disadvantageous in this method that the aging processes and the overall loss of capacitance resulting therefrom are determined without adequately including an instance of predamage to the secondary cell. Furthermore, no account is taken of damages owing, in particular, to events such as large changes in a state of charge. The result of this is an excessively low overall loss of capacitance which does not adequately reflect reality.